This project will develop and evaluate a theory-based behavioral intervention to modify diet and physical activity and reduce risk for obesity, cancer and other chronic diseases in children aged 10 to 12 years recruited through a managed care setting. Managed care provides an ideal setting for reaching families and for disseminating effective obesity prevention programs. The development of the intervention will be completed using established theories in health behavior (e.g., Social Cognitive Theory, Self-Determination Theory), research on the predictors of diet and physical activity in children, and knowledge gleaned from prior tailored print intervention studies. Intervention components will be developed over a 9-month period and will receive targeted testing during development. The OPT for Health II intervention will be evaluated using a randomized experimental design in which families (N=400) will be recruited from the KPSC membership database, matched on single or dual parent household status and number of children living in the home, and randomly assigned to the OPT for Health II intervention condition or a usual care control condition. Families in the control condition will receive usual care in the KPSC system which includes notification and access to classes on diet, physical activity and weight control. Randomization of families will occur after the baseline assessment is completed to eliminate measurement bias based on treatment assignment. Measures of diet, physical activity, psychosocial mediators, height and weight (Body Mass Index) will be completed on the recruited target parent and child in each family. Assessments will occur at clinic sites of KPSC, a major health maintenance organization in Southern California and will be completed by trained interviewers. The OPT for Health II intervention will include (a) one meeting between the participating families and a health educator using MI principles to motivate families for change, to select an initial behavior to change, and to develop an action plan, (b) four tailored newsletters (2 for diet, 2 for physical activity) for parents and four tailored newsletters for the target children that are also culturally targeted (Hispanic, Non-Hispanic), (c) two family activities (1 for diet, 1 for physical activity) for both parents and children with instructions included as part of the tailored newsletters, and (d) five Motivational Interviewing telephone calls (2 diet, 2 physical activity, 1 summary) for parents. These intervention operations have been tested for feasibility and for initial efficacy estimates using R21 funding from the National Cancer Institute (1R21CA95016). In OPT for Health II, recruitment will occur over a 20 month period using a rolling recruitment and intervention strategy. Five families will be recruited weekly with baseline assessment completed shortly after recruitment. Randomization to conditions will then occur and the intervention will be delivered over a 24-week period (6 months). Follow-up assessments will occur at 6- and 12-months post-baseline and will provide outcome measures and measures of potential mediators for observed changes in diet, physical activity and BMI. Two 24-hour recalls will be completed at each assessment period. BMI and physical activity will be objectively measured through height and weight assessments, and through the use of accelerometry to assess physical activity.